feat: [#1] add @xedo/sdk node client for developer api v1

[nahtandev]

Summary

Introduces @xedo/sdk, the official server-side Node.js/TypeScript client for the
Xedo Developer API v1, so e-commerce/SaaS templates share one typed, resilient API
layer instead of duplicating auth, retries, pagination and error handling.

Changes

• Added Xedo client (src/client.ts): configurable apiKey, baseUrl,
  maxRetries, timeoutMs, injectable fetch, browser-env guard, environment
  and lastRateLimit getters
• Added a fetch transport (src/transport.ts): Bearer auth, envelope unwrapping,
  automatic 429 retry (Retry-After + backoff/jitter, no 5xx auto-retry on
  POST /v1/carts), rate-limit capture, per-request timeout/abort, camelCase →
  wire (per_page, _sort, _order) mapping
• Added the 5 resources ping, products, collections, orders, carts
  including pagination (list + listAll async iterator), binary invoice PDF, and
  the carts.createAndPay 502 checkout helper
• Added typed error hierarchy (src/errors.ts) routed on code (XedoAuthError,
  XedoValidationError, XedoNotFoundError, XedoStockError, XedoConflictError,
  XedoRateLimitError, XedoPaymentInitError, XedoConnectionError)
• Added hybrid types: generated mirror via openapi-typescript plus a hand-written
  public surface (src/types/public.ts)
• Added vitest + msw tests (16 cases: success, errors, retry, pagination, invoice,
  createAndPay), tsup ESM+CJS+.d.ts build, eslint flat config, and GitHub
  Actions CI on Node 18/20/22
• Added README, Next.js Server Component and Node script examples